This invention relates generally to rotation sensors and parficularly to ring laser gyroscope rotation sensors. Still more particularly, this invention relates to apparatus and methods for supporting a ring laser gyroscope frame to allow rotational motion about the input axis of the ring laser gyroscope to reduce the effects of lock-in and to provide extremely high stiffness for all other degrees of freedom to reduce coning errors.
A ring laser gyroscope employs the Sagnac effect to detect rotation. Two counterpropagating light beams in a closed loop will have transit times that differ in direct proportion to the rotation rate of the loop about an axis perpendicular to the piane of the loop. There are in general two basic techniques for utilizing the Sagnac effect to detect rotations. A first technique is the interferometric approach, which involves measuring the differential phase shift between two counterpropagating beams injected from an external source, typically a laser, into a Sagnac ring. The ring may be defined by mirrors that direct the light beams around the path or by a coil of optical fiber. Beams exiting the path interfere and create a pattern of iight and dark lines that is usually called a fringe pattern. Absolute changes in the fringe pattern are indicative of rotation of the ring. The primary difficulty with such devices is that the changes are very small for rotation rates of interest in guidance applications.
The ring laser gyroscope uses the resonant properties of a closed cavity to convert the Sagnac phase difference between the counter propagating beams into a frequency difference. The high optical frequencies of about 10.sup.15 Hz for light used in ring laser gyroscopes cause the minute phase changes to become beat frequencies that are readily measured.
A ring laser gyroscope has a sensor axis that passes through the closed paths traversed by the counterpropagating beams. When the ring laser gyroscope is not rotating about its sensor axis, the optical paths for the two counterpropagating beams have identical lengths so that the two beams have identical frequencies. Rotation of the ring laser gyroscope about its sensor axis causes the effective path length for light traveling in the direction of rotation to increase while the effective path length for the wave traveling opposite in direction to the rotation decreases.
Ring laser gyroscopes may be classified as passive or active, depending upon whether the lasing, or gain, medium is external or internal to the cavity. In the active ring laser gyroscope the cavity defined by the closed optical path becomes an oscillator, and output beams from the two directions beat together to give a beat frequency that is a measure of the rotation rate. The oscillator approach means that the frequency filtering properties of the cavity resonator are narrowed by many orders of magnitude below the passive cavity and give very precise rotation sensing potential. To date the major ring laser gyroscope rotation sensor effort has been put into the active ring laser. Presently all commercially available optical rotation sensors are active ring laser gyroscopes.
When the rotation rate of the ring laser gyroscope is within a certain range, the frequency difference between the beams disappears. This phenomenon is called frequency lock-in, or mode locking, and is a major difficulty with the ring laser gyroscope because at low rotation rates the ring laser gyroscope produces a false indication that the device is not rotating. If the rotation rate of a ring laser gyroscope starts at a value above that where lock-in occurs and is then decreased, the frequency difference between the beams disappears at a certain input rotation rate. This input rotation rate is called the lock-in threshold. The range of rotation rates over which lock-in occurs is the deadband of the ring laser gyroscope.
Lock-in is beiieved to arise from coupling of light between the beams. The coupling results primarily from backscatter off the mirrors that confine the beams to the closed path. Backscatter causes the beam in each direction to include a small component having the frequency of the beam propagating in the other direction. The lock-in effect in a ring laser gyroscope is similar to the coupling that has been long been observed and understood in conventional electronic oscillators.
Upon reversal of the sign of the frequency difference between the two beams, there is a tendency for the beams to lock-in since at some point the frequency difference is zero. Since the output of the ring laser gyroscope is derived from the frequency difference, an error accumulates in the output angle. Since the periods in which the two beams are locked in are usually very short in duration, the error is very small. However, since the error is cumulative, in time the error can become appreciable in precision navigation systems. This error is called random walk or random drift.
In addition to causing erroneous rotation rate information to be output from a ring laser gyroscope, lock-in causes standing waves to appear on the mirror surfaces. These standing waves may create a grating of high and low absorption regions, which create localized losses that increase the coupling between the beans and the lock-in. The mirrors may be permanently distorfed by leaving a ring laser gyroscope operating in a lock-in condition.
Any inability to accurately measure low rotation rates reduces the effectiveness of a ring laser gyroscope in navigational systems. There has been substantial amount of research and development work to reduce or eliminate the effects of lock-in to enhance the effective use of ring laser gyroscopes in such systems.
There are several known approaches to solving the problems of lock-in. One such approach involves mechanically oscillating the ring laser gyroscope about its sensor axis so that the device is constantly sweeping through the deadband and is never locked therein. This mechanical oscillation of the ring laser gyroscope is usually called dithering. A typical ring laser gyroscope may be dithered at about 400 Hz with an angular displacement of a few arc minutes.
Mechanical dithering is accomplished by mounting the ring laser gyroscope frame on a flexure device that includes a plurality of vanes or blades extending from a central portion. Each blade has a pair of piezoelectric elements mounted on opposite sides thereof. Voltages are applied to the piezoelectric elements such that one piezoelectric element on each blade increases in length while the other piezoelectric element decreases in length. The effect of these length changes in the piezoelectric elements is transmitted to the blades through the mounting of the piezoelectric elements thereon. Increasing the length of one side of each blade while shortening the other side causes the blades to flex or bend so that each blade experiences a small rotation about the ring laser gyroscope axis. The voltage is oscillatory so that the blades are constantly vibrating in phase, and the ring laser gyroscope frame mounted to the blades rotates about the axis.
Body dither must be accomplished so that dither oscillations cause the ring laser gyroscope frame to rotate only about the sensing axis. Any small component of rotation about other axes causes the sensing axis to precess in a cone-shaped path about the direction in should point. This motion of the axis is called coning. Any change in the direction of the axis due to dithering introduces errors into the output of the ring laser gyroscope. Since a navigation system includes three ring laser gyroscopes mounted in an instrument block with the sensing axes being mutually orthogonal, mechanical coupling of the dither oscillations is likely.
To reduce coning, the plane of oscillation of the flexure is aligned perpendicular to the sensing axis, and the axis of the dither is collinear with the sensing axis to very close tolerances. To further minimize oscillations about other axes, the dither flexure should be as rigid as possible to resist any tendency to oscillate about other axes. Since all mechanical systems have natural frequencies of oscillation, there will in general be some small amount of osciilation about other axes. Typical prior art dither flexures have rotational and translational resonant frequencies below 1000 Hz and have relatively high compliances, which, when combined with relatively low coning frequencies, lead to large system bias errors. These compliant flexures allow a relatively large amplitude frame input axis motion, which couples with system block motion to cause angle errors that cannot be software compensated.
U.S. Pat. No. 4,115,004 to Hutchings et al., assignee to Litton Systems, Inc., assignee of the present invention, discloses a dual spring system that mounts a counterweight to the ring laser gyroscope case to reduce oscillatory motion of the case due to oscillation of the gyroscope. This dual spring system includes a first set of springs mounted between the case and the gyroscope and a second set of springs mounted between the case and the counterweight.
U.S. Pat. No. 4,309,107 to McNair et al., assignee to Litton Systems, Inc., discloses a ring laser gyroscope dither mechanism for isolating vibrational energy generated by dithering the gyroscope and prevents that energy from passing to the mounting case of the gyroscope. McNair et al. discloses a three spring system mounting a gyroscope to a housing or case, mounting a counterweight to the gyroscope and mounting the counterweight to the case. This arrangement reduces the amount of angular vibrational energy that passes to the case of the gyroscope by using the counterweight to provide a reaction to the oscillations within the gyroscope caused by mechanicaliy dithering to prevent lock-in.
U.S. Pat. No. 4,321,557 to McNair, assignee to Litton Systems, Inc., discloses a ring laser gyroscope coupling system in which a pair of resilient rings are located between a plate attached to the laser dither suspension mechanism and the lower surface of the case of the ring laser gyroscope to form a reservoir for a viscous fluid. The viscous fluid reduces transmission of thermal stresses between the case and the dither suspension mechanism.
U.S. Pat. No. 4,349,183 to Wirt et al., assignee to Litton Systems, Inc., discloses a spring flexure assembly for a ring laser gyroscope dithering mechanism. The assembly includes a plurality of flexure springs radially extending between a hub and a rim with each spring being driven by four piezoelectric wafers. Each spring has a reduced moment of inertia about an axis parallel to the common axis of the rim and hub and an increased circumferential bending in the region of attachment to the rim.
U.S. Pat. No. 3,464,657 to Bullard discloses a single set of springs connected between the frame and mounting platform of an aircraft instrument to isolate vibrational energy from the instrument.
U.S. Pat. No. 3,373,650 to Killpatrick discloses a dithering system in which a varying bias in the frequency is applied to at least one of the counterpropagating beams. The varying bias causes a varying frequency difference between the counterpropagating beams. This frequency difference is generally greater than the frequency difference that occurs at the lock-in threshold. The polarity of the frequency difference is periodically alternated so that the time integral of the frequency difference over the time interval between sign reversals is substantially zero.
U.S. Pat. No. 4,436,423 to Kumar et al. discloses a ring laser gyroscope suspension comprising a torsional hinge axially mounted within a central bore of a ring laser gyroscope body. The hinge comprises a plurality of angularly spaced wing sections having radially extending slits for permitting torsionai motion of the gyroscope about the hinge. A piurality of spaced, curved segments bridge a gap between the wing sections and and the confronting surface of the bore and are cemented thereto for securement to the hinge.
Previous ring laser gyroscope dither flexures are mounted to the frames by relatively flexible bonding agents such as polyurethane glue to absorb thermal stresses to prevent thermal fluctuations from causing problems such as changing the path length or misaligning the counterpropagating beams in the resonant cavity. Misalignment reduces the power output of the ring laser gyroscope. Changes in the path length change the resonant wavelength of the resonant cavity and the wavelength at which the lasing medium will provide gain. Changing the wavelength changes the scale factor of the ring laser gyroscope, which is undesirable during operation thereof since the accuracy of measurements provided by the ring laser gyroscope requires a known, constant scale factor.
The excessive flexibility of prior methods for mounting dither flexures in ring laser gyroscope frames increases coning errors and lowers the resonant frequency of the dither flexure assembly. Typical prior art dither flexures have resonant frequencies of about 400-700 Hz. Since the ring laser gyroscope is dithered at about 300-500 Hz about the sensor axis, the dither vibrations about one axis are transmitted to the other axes. The resonant frequencies of these prior dither flexures are sufficiently close to the dither frequency that the dither drive also causes oscillations about axes of other ring laser gyroscopes that are typicaliy included in a guidance system.
Prior dither flexures fit in a generally cylindrical cavity in the frame, and the outer surface of the dither flexure must be precisely machined and aligned concentrically with the cylindrical cavity. A uniform space must be provided between the dither flexure and the walls of the cylindrical cavity for receiving the flexible glue. Any misalignment of the components or nonuniformity in the gap will cause stress points in the frame, and such stress points may severely diminish the performance of the ring laser gyroscope.